This section provides background information related to the present disclosure which is not necessarily prior art. Lean burn engines provide improved fuel efficiency by operating with an excess of oxygen, that is, a quantity of oxygen that is greater than the amount necessary for complete combustion of the available fuel. Such engines are said to run “lean” or on a “lean mixture.” However, this improved or increase in fuel economy, as opposed to non-lean burn combustion, is offset by undesired pollution emissions, specifically in the form of oxides of nitrogen (NOx).
One method used to reduce NOx emissions from lean burn internal combustion engines is known as selective catalytic reduction (SCR). SCR, when used, for example, to reduce NOx emissions from a diesel engine, involves injecting an atomized reagent into the exhaust stream of the engine in relation to one or more selected engine operational parameters, such as exhaust gas temperature, engine rpm or engine load as measured by engine fuel flow, turbo boost pressure or exhaust NOx mass flow. The reagent/exhaust gas mixture is passed through a reactor containing a catalyst, such as, for example, activated carbon, or metals, such as platinum, vanadium or tungsten, which are capable of reducing the NOx concentration in the presence of the reagent.
An aqueous urea solution is known to be an effective reagent in SCR systems for diesel engines. However, use of such an aqueous urea solution involves many disadvantages. Urea is highly corrosive and may adversely affect mechanical components of the SCR system, such as the injectors used to inject the urea mixture into the exhaust gas stream. Urea also may solidify upon prolonged exposure to high temperatures, such as temperatures encountered in diesel exhaust systems. Solidified urea will accumulate in the narrow passageways and exit orifice openings typically found in injectors. Solidified urea may also cause fouling of moving parts of the injector and clog any openings or urea flow passageways, thereby rendering the injector unusable.
In addition, if the urea mixture is not finely atomized, urea deposits will form in the catalytic reactor, inhibiting the action of the catalyst and thereby reducing the SCR system effectiveness. High injection pressures are one way of minimizing the problem of insufficient atomization of the urea mixture. However, high injection pressures often result in over-penetration of the injector spray plume into the exhaust stream, causing the plume to impinge on the inner surface of the exhaust pipe opposite the injector. Over-penetration also leads to inefficient use of the urea mixture and reduces the range over which the vehicle can operate with reduced NOx emissions. Only a finite amount of aqueous urea can be carried on a vehicle, and what is carried should be used efficiently to maximize vehicle range and reduce the need for frequent replenishment of the reagent.
Several known reagent injectors include a solenoid valve for metering the supply of reagent into the exhaust stream. Typically, a magnetic moveable member of the valve is urged to translate between open and closed positions as an electromagnet is selectively energized and deenergized. The electromagnets of many prior injectors include multiple flux leakage areas resulting in a poorly defined magnetic circuit. Control of the reagent valve may not be optimized using these types of magnetic circuits. The amount of reagent actually dispensed within the exhaust system may vary from a target rate of reagent injection resulting in inefficient use of the onboard reagent. The time required for the valve to cycle from a closed condition, to an opened condition, and back to a closed condition, may be larger than desired due to the magnetic circuit arrangement.
Further, aqueous urea is a poor lubricant. This characteristic adversely affects moving parts within the injector and requires that relatively tight or small fits, clearances and tolerances be employed between adjacent or relatively moving parts within an injector. Aqueous urea also has a high propensity for leakage. This characteristic adversely affects mating surfaces requiring enhanced sealing resources in many locations.
It may be advantageous to provide an improved electromagnetically controlled injector having a well defined magnetic circuit to improve reagent injection control.
Methods and apparatus of the present disclosure provide the foregoing and other advantages.